The season 6 mechanic is once again very boring, you just get 15% exp and occasionally you get some drops. The undercity feels kinda underwhelming as exclusive xpac activity.
However the undercity is something that would have been an excellent seasonal mechanic. It offers something different for current dungeons by introducing time and target rewards. Has new bosses, with actual mechanics that could rotate into dungeons after season. You can alter the rewards to your liking by offering up some materials.
With the constant overhauling of the game the past year each season was more about the changes than actual seasonal mechanics. But with probably most big changes behind us they cant feed us the same lame seasonal mechanics to keep us coming back.